The Ottawa Embayment contains erosional remnants of a shallow-water carbonate platform (St. Lawrence Platform) of Late Ordovician (Caradocian) age. Stratigraphy of three Blackriveran formations — in ascending order, Pamelia, Lowville, and Chaumont — documents regional changes in continuity and types of depositional facies. The Pamelia Formation contains two members, each containing basin-wide basal siliciclastic units overlain by interbedded limestone, dolostone, and shale. An alternate division recognizes six shallowing-upward units; regionally extensive, intertidal to supratidal, dolomitic sandstone and (or) sandy dolostone define their tops. Sedimentary structures and isotope (C, O) geochemistry support a syngenetic origin of this stratigraphic (bedded) dolomite. The Lowville Formation contains two facies associations: subtidal to lower intertidal bioclastic and oolitic packstone – grainstone followed by lagoonal to intertidal mudflat facies. Lateral facies continuity is reduced compared to the Pamelia Formation. The Chaumont Formation contains thick beds of burrowed, bioclastic, peloidal mudstone to packstone, and minor shale. No rhythmic pattern is recognized in these subtidal facies. Upsection decrease of rhythmic sedimentation, with a decrease in lateral facies continuity of the studied strata reflects a progressive increase in net accommodation space related to Taconic transgression. Higher order rhythmicity of dolostone and sandstones of the Pamelia Formation can be traced into adjacent regions (New York and Kingston, Ont.). Dolomitic units may identify basin-wide chronostratigraphic markers, potentially useful for future sequence analysis. Regional correlation reveals a good oceanographic linkage between the Ottawa Embayment and the Appalachian Basin during Pamelia time and a restricted access across a paleohigh in the Montréal region. By the time of Lowville deposition, Taconic transgression had breached this restriction.